A microwave plasma processing apparatus known in the related art uses high density plasma excited by a microwave electric field. For example, the microwave plasma processing apparatus has a processing container configured to plasma-process a substrate to be processed, a microwave generator configured to generate microwaves to generate plasma of a processing gas in the processing container, and a waveguide configured to guide the microwave generated by the microwave generator into the processing container. Further, the microwave plasma processing apparatus includes a planar antenna having a plurality of slots that transmit the microwaves guided by the waveguide. In the microwave plasma processing apparatus, the microwaves are radiated into the processing container from the slot antenna and the processing gas in the processing container is deionized to excite the plasma.
However, in such a microwave plasma processing apparatus, it is requested that the plasma excited by the microwaves be uniformly distributed in processing space of the processing container in order to ensure uniform plasma processing of the entire processed surface of a workpiece. In this regard, there is a known technology in which a distributor is installed at a rear end side of the microwave generator to distribute the microwaves from the microwave generator to a plurality of waveguides by the distributor at a predetermined distribution ratio and radiate the distributed microwaves to a center side and a periphery side of the processing space from the antenna. See, for example, Japanese Patent Laid-Open Publications Nos. H9-63793, H3-191074, and 2007-213994.